The [cmake documentation of `CXX_STANDARD_REQUIRED`](https://cmake.org/cmake/help/v3.5/prop_tgt/CXX_STANDARD_REQUIRED.html)
demonstrate that this was previously incorrectly set. This forces the
compiler flag to be specified. In the case of gcc 4.8 it sets
`-std=c++1y`. Newer gcc (since version 5) will sets `-std=c++14`.